Similarity.

UNLABELLED: Similarity is a fundamental concept within Cognitive Science. It is routinely invoked in the explanation of cognitive processes as diverse as memory retrieval, categorization, visual search, problem solving, learning, language processing, reasoning, and social behavior. At the same time, it is of fundamental practical concern to computer scientists concerned with clustering and machine learning, and it figures in many philosophical contexts. Crucially, 'similar' is not a relationship that simply reflects objective properties of the objects under consideration but rather is dependent on how those objects are represented by an observer. This ties theories of similarity closely to theories of representation. For further resources related to this article, please visit the WIREs website. CONFLICT OF INTEREST: The authors have declared no conflicts of interest for this article.